According to such a method, a lignocellulose-based raw material is cooked in an alkaline cooking liquor which comprises, besides conventional cooking chemicals of sulphate cooking, also polysulphide and antraquinone.
An alkaline cooking liquor which comprises polysulphide is often called orange liquor. Hereinafter, when associated with the present invention, this term is used as a synonym of alkaline cooking liquor which comprises polysulphide.
Use of polysulphide (PS) in combination with antraquinone (AQ) in sulphate pulp cooking, i.e. “PSAQ cooking”, is a method which is known and applied industrially. PSAQ cooking is used in about ten factories in the world. PSAQ cooking is particularly suitable to be used together with traditional batch cooking and continuous cooking, because in traditional cooking all the white liquor is dosed at the beginning of the cooking process. In this case, it is possible to fully exploit particularly the effect of increasing the hemicellulose yield of PS at a low temperature in the absorption stage of the cooking.
In the case of modified cooking, the white liquor is dosed in several steps into the cooking process. As in traditional cooking, the first dosing point is at the beginning of the cooking, under low temperature conditions (<120° C.). However, modified cooking is characterised by a substantial part of the alkaline cooking liquor being dosed as hot or in conditions which can easily bring the dose to the cooking temperature, i.e. at a temperature of above 140° C., which decomposes the PS rapidly. This means that the effect of increasing the yield of the PS cannot be exploited as efficiently in modified cooking methods as in traditional cooking, because only part of the alkaline cooking liquor which comprises PS, i.e. of the orange liquor, can be dosed in conditions of low temperature, which are required to increase the yield.
Methods which aim at utilising the PS as efficiently as possible and in a way which suits the process in question in an optimal way have been developed for use in combination with modified cooking.
For example, Metso has filed a patent application for a method by which PSAQ cooking is carried out in a Super Batch batch cooking process (EP 1702101). The same company also has a method of using polysulphide in modified continuous cooking (PCT-patent application WO 2003/057979).
In neither of the Metso methods is it possible to fully exploit the PS effect because several tens of per cents of the alkaline cooking liquor batch are consumed in the actual cooking stage.
Patent application US 2009/0126883, in International Paper (hereinafter “IP”), describes different ways of applying PSAQ cooking in modified continuous cooking. The solution which is described in the IP patent application differs from previous methods associated with PS cooking in that it is possible to dose the entire white liquor dose, which comprises PS, at the beginning of the cooking thereby maximising the improvement in yield. This is enabled by arranging the cooking liquors in such a way that liquors are replaced one by another and the removed liquors are used in later stages of the process.
One liquor is taken out of the boiler but a totally different liquor is brought back. In other words, the liquor which is taken out is directed to a different part of the boiler or removed from the cooking process and, correspondingly, the liquor which is brought back is sourced from a different part of the total process. Consequently, the liquor circulations become in practice very complicated when executed according to the solution in IP.